Abstract

This thesis presents an investigation of the selective growth of carbon nanotubes by examining the chemical and physical interaction of the catalyst with various catalyst supports, namely, Al, Ti and SiO2. Numerous surface analysis techniques including AFM, XPS, FESEM, TEM, and Raman spectroscopy have been employed to ascertain that the observed changes in nanotube growth for this system are related primarily to changes in the catalyst support. It is demonstrated that the nucleation density of single-walled carbon nanotubes, formed by thermal catalytic chemical vapor deposition, strongly depends on the grain size of Al catalyst supports covered with a native oxide (Al/Al2O3). By varying the substrate temperature during Al sputter deposition it was possible to investigate the effect of Al grain size on growth without inducing changes in the catalyst support thickness, surface chemistry, or any other growth parameter. Furthermore, it is also shown that by altering the degree of oxidation of physically deposited Al and Ti catalyst supports, the nucleation density of filamentary carbonaceous deposits, such as carbon nanotubes and carbon nanofibres, can be controlled. Using SiO2 as a catalyst support, the growth time, growth temperature, anneal time, carbon feedstock flow rate, and the effect of water vapour on the growth of CNTs was investigated. By varying these parameters, it was demonstrated that the number of active nucleation sites could be changed. Moreover, the unsuitability of physically deposited Ni catalyst on Si/SiO2 substrates to grow horizontally-bound ultralong CNTs, and vertically-aligned ultralong CNTs, using water vapour is also discussed.
Two thermal CVD approaches, namely, heated-wall and localised-heating furnace designs, were used to synthesis carbon nanotubes. The heated-wall CVD system was a large 10 cm diameter by 180 cm long tubular furnace. The localised-heating system was comprised of a local source of heat that was encased in a 5 cm diameter by 35 cm long tube. Two separate methods of localised-heating were developed. The first method used a resistively-heated 1 cm wide by 4 cm long highly-doped silicon bridge, which was referred to as the cold-wall furnace. The second method used microheaters that were made of NiCr and Pt. It is shown by use of numerical modelling of both types of furnace designs that, depending on the design of the furnace, different gas flow schemes can occur, which will affect the concentration of gases and carbon feedstock during CNT growth.
Using the cold-wall furnace, a simple novel approach was developed for rapid evaluation of carbon nanotube growth that permitted the investigation of a wide range of temperatures in one experimental cycle. A series of experiments, using alcohol-assisted thermal CVD, are shown to demonstrate the effectiveness of such an approach. Pt-based microheater designs are shown to successfully synthesise filamentous carbonaceous deposits. Future microheater devices are discussed with reference to characterisation methods and potential device concepts.

Item Type:

Thesis
(PhD)

Qualification Level:

Doctoral

Additional Information:

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